1. Technical Field
The present invention relates to spindle motors; in particular to disk-drive spindle motors employing hydrodynamic bearings that generate rotational dynamic pressure in shaft-sleeve interposed lubricant for rotationally bearing the disk-supporting shaft/sleeve.
2. Description of Related Art
Disk drive devices, such as computer hard disk drives, in which spindle motors are employed to drive data-storing disks, are well known. Spindle motors of this type may include hydrodynamic bearing configurations that generate bearing pressure in the lubricating fluid dynamically when the motor spins, thereby stabilizing, for example, a disk-supporting sleeve rotationally against a stationary spindle shaft.
One such motor is disclosed in U.S. Pat. No. 5,504,637. The disclosed motor includes a stationary shaft, a thrust plate fixed endwise to the stationary shaft, and a rotary hub having an annular central recess encompassing the thrust plate and integral with a sleeve surrounding the shaft. The motor further includes a thrust washer fixed over the central recess in the hub, confining the thrust plate in the rotary hub recess. Lubricant is retained in a clearance between the base of the recess and the axially adjacent surface of the thrust plate; the lubricant and the clearance-defining surfaces of the recess and thrust plate form a first hydrodynamic thrust bearing. Axially adjacent, clearance-defining surfaces of the thrust plate and thrust washer, together with lubricant retained in the clearance form a second hydrodynamic thrust bearing.
Herringbone grooves for generating hydrodynamic pressure are formed in a first portion of the cylindrical surface of the shaft. The first portion of the shaft cylindrical surface is surrounded by the radially adjacent inner circumferential surface of the sleeve at an annular micro-gap filled with lubricant. The grooved first portion of the shaft cylindrical surface, the adjacent inner surface of the sleeve, and the lubricant in the micro-gap establish a first radial bearing. Hydrodynamic-pressure-generating herringbone grooves are also formed in a second portion of the shaft cylindrical surface, radially adjacent the inner surface of the sleeve at another annular micro-gap filled with lubricant. The grooved second portion of the shaft cylindrical surface, the adjacent inner surface of the sleeve, and the lubricant in the micro-gap establish a second radial bearing. The hydrodynamic-pressure-generating grooves in the first and second radial bearings generate hydrodynamic pressure when the sleeve rotates relative to the shaft.
The hydrodynamic pressure thus generated in the radial bearings imparts high rigidity to the radial bearings to stabilize sleeve rotation. To stabilize sleeve rotation further, the first and second radial bearings are spaced apart at a predetermined distance, supporting the sleeve to eliminate wobble as it rotates about the shaft.
In a conventional motor of the foregoing type, the first and second radial bearings provide radial stability to the sleeve, maintaining the rotary hub in a vertical orientation with respect to the stationary shaft. Further, the first and second radial bearings maintain the sleeve in a concentric relationship with respect to the stationary shaft during rotation of the rotary hub. The effectiveness of the radial bearings in maintaining the rotary hub in a constant concentric relationship with respect to the shaft depends on the rigidity of each of the radial bearings and the axial distance between their respective centers. The farther apart the first and second radial bearings are, the more stable the rotary hub rotation will be against radial movement with respect to the shaft, since the thrust bearings primarily only restrain axial movement of the rotary hub.
Personal computers, in which disk-drive storage devices driven by conventional motors such as described above are utilized, are continually becoming smaller and thinner. The motors for spinning the hard disk in these disk-drive storage devices are expected to become smaller and thinner as well. Because the radial bearings are essential to the radial support of the rotary hub, however, and because making the distance between the radial bearings as large as possible is advantageous for imparting greater rotational stability to the rotary hub, reducing the axial height of motors employing first and second radial bearings presents difficulties.
Moreover, simply making motor structural components, such as the rotary hub and the thrust plate, thinner in order to reduce the motor axial height makes secure, precision assembly of the motor components to one another difficult. In particular, if the shaft and the thrust plate are not securely fixed to one another, the rotational precision of the motor is negatively affected.
Japanese laid patent application 08331796 (1996) discloses a different type of motor that includes a stationary sleeve encompassing a rotational shaft. In this case as well two axially separated sets of hydrodynamic-pressure-generating grooves are formed in the cylindrical surface of the shaft. The grooved sections of the shaft cylindrical surface and radially adjacent sections of the inner circumferential surface of the stationary sleeve define annular micro-gaps in which lubricant is retained, and together with the lubricant form upper and lower radial hydrodynamic bearings for supporting the rotational shaft.
However, the motor configuration disclosed in this Japanese publication does not utilize the radially extensive surface(s) of a thrust plate to establish hydrodynamic thrust bearing(s) as, in contrast, does the first motor configuration discussed above. Rather, hydrodynamic pressure generation grooves formed on the base-end surface of the shaft and/or the adjacent surface of a plate fixed to the sleeve, and lubricant in the clearance defined between the two surfaces, form a single hydrodynamic thrust bearing. Since the shaft is of diameter that is proportionately smaller than the thrust plate in the first motor discussed above, the grooves formed on the base-end surface of the shaft and/or the adjacent surface of the plate may not be able to generate sufficient thrust hydrodynamic pressure to support adequately the thrust load generated by rotation of the motor. Increasing the diameter of the shaft in order to obtain increased hydrodynamic pressure in the lubricant in the thrust bearing is not a practical consideration for this motor, because an increased shaft diameter in such a motor would result in greater energy loss that would decrease the electrical efficiency of the motor.
U.S. Pat. No. 5,659,445 to Yoshida et al. is directed to improving the lubricating configuration in a recording disk-drive motor. As set forth in the Summary section, Yoshida et al. accomplish this i) by employing tapered lubricant clearances in the thrust and radial dynamic-pressure bearing sections to increase dynamic lubricant pressure, and ii) by containing the dynamic pressure lubricant with a magnetic fluid seal device. Yoshida et al. thus seek to improve bearing and lubricating performance by increasing the dynamic pressure generated in, and at the same time keeping air out of, the radial and thrust bearing sections. The magnetic fluid seal device taught by Yoshida et al. is to prevent air, which has a larger coefficient of thermal expansion/contraction than the lubricant, from entering the bearing sections and destabilizing their performance.
Yoshida et al. thus teach improving motor bearing performance by employing tapered clearances to increase rotational dynamic pressure in the lubricant, which at the same time necessitates containing the lubricant with magnetic fluid seal devices. In turn, the magnetic fluid seal devices taught in every pertinent Yoshida et al. embodiment require a separate thrust plate/member for at least the thrust bearing on the rotor-hub adjacent end.
For example, Yoshida et al. discloses, as shown in FIG. 18, a recording-disk rotating device that includes a radial bearing portion 117 and thrust bearing portions 118 and 119. The radial bearing portion 117 is constituted by a combination of the inner circumferential surface of the circular hole in the bush 107 and helical or herringbone grooves 131 on the cylindrical surface of the shaft 105. The thrust bearing portions 118 and 119 are constituted by a combination of both the flat axial-end surfaces of the bush 107 and the respectively adjacent thrust plate 122 and thrust member 121, as well as tapered grooves 130 or spiral grooves 132.
Accordingly, the Yoshida et al. configuration employs dual thrust bearing portions 118 and 119. Consequently, bearing losses due, for instance, to fluid friction in the lubricant filling the clearances formed in both the thrust bearing portions 118 and 119 may be large, which lowers the electrical efficiency of the motor.
Furthermore, the necessity of the thrust plate/member in the recording disk rotating device as taught by Yoshida et al. significantly limits the amount by which the axial height can be reduced. For example, in the Yoshida et al. embodiment described above, the configuration of the thrust bearings 118 and 119 requires the thrust member 121 on the rotor hub 104 end of the shaft 105, and employs a thrust plate 122 as well. The magnetically conductive thrust member 121 is required to form a magnetic fluid seal device together with the magnet assembly 125.
Yoshida et al. employ tapered sections within the dynamic-pressure generating portions of the lubricant clearances to increase hydrodynamic pressure; thus instead of using taper seals along the lubricant boundaries to prevent leakage, special magnetic seals requiring at least one extra axially disposed part a thrust plate/member are used to magnetically seal a magnetic fluid as the dynamic-pressure generating lubricant in the thrust bearings.
The hydrodynamic bearings in the motor taught by Yoshida et al. are constituted by four components, namely: the shaft 105, the bush 107, the thrust member 121, and the thrust plate 122. Further, the hydrodynamic bearings include bearing gaps defined by six surfaces, namely: the outer circumferential surface of the shaft 105; the inner circumferential surface, and the axial upper and lower surfaces, of the bush 107; the axial lower surface of the thrust member 121; and the axial upper surface of the thrust plate 122.
In general, bearing surfaces that define hydrodynamic bearing gaps must be precisely machined to close tolerances, and the hydrodynamic bearing components must be precision-assembled, which makes hydrodynamic bearing manufacturing costs high. Accordingly, the number of bearing surfaces and parts desirably should be reduced to cut down motor manufacturing costs.
In view of the foregoing there exists a need for a spindle motor that overcomes the prior art problems mentioned above.
An object of the present invention is to reduce the axial height of and otherwise make smaller a spindle motor.
Another object is to facilitate the assembly of such a smaller and thinner spindle motor in order to minimize production costs.
Still another object is to configure a bearing structure for such a smaller and thinner spindle motor to provide the motor with a high level of rigidity.
Yet another object is to reduce the axial height of and make smaller a spindle motor by eliminating hydrodynamic bearing thrust plates entirely from the motor configuration, and yet to maintain thrust-load support in such a smaller and thinner spindle motor and make its manufacture and assembly simpler and less costly.
Another object of the present invention is to reduce the axial height of and otherwise make smaller a disk-drive device.
A further object is to facilitate and make less costly the assembly of such a smaller and thinner disk-drive device.
Still another object is to configure a bearing structure for such a smaller and thinner disk-drive device to provide the device with a high level of rigidity.
A yet further object is to reduce the axial height of and make smaller a disk-drive device by eliminating hydrodynamic bearing thrust plates entirely from the hydrodynamic thrust bearing configuration, to make manufacture and assembly of the disk-drive device simpler and less costly.
In accordance with one aspect of the present invention, a disk-drive motor is configured to rotate on a hydrodynamic radial bearing and a magnetically counterbalanced single hydrodynamic thrust bearing. The motor includes a support cylinder defining a central bore and a shaft coaxially inserted and extending at least partially into the support cylinder bore. An axially extending micro-gap is defined radially between the shaft circumferentially and the bore. A rotor hub is fixed axially endwise to the shaft, and the rotor hub itself constitutes a circular inner face opposing the support cylinder endwise. A radially extending micro-gap is defined axially between the circular inner face of the rotor hub and the end of the support cylinder. From the outer circumference of the rotor hub a cylindrical wall extends coaxially with the shaft, encompassing the stator, and a rotor magnet is fixed to the inner margin of the cylindrical wall, opposing the stator. Lubricant fills the axially and radially extending micro-gaps.
A radial-hydrodynamic pressure bearing including the lubricant-filled axially extending micro-gap, and hydrodynamic pressure-generating grooves formed in either the circumferential surface of the shaft or the bore, is thus established in this configuration. One single thrust-hydrodynamic pressure bearing including the lubricant-filled radially extending micro-gap, and hydrodynamic pressure-generating grooves formed in either the circular inner face of the rotor hub or the end of the shaft-support ring is also thus established in this configuration.
Further, magnetic counterbalancing means associated with the cylindrical wall of the rotor hub are provided for generating magnetically attractive force attracting the rotor hub axially toward the shaft-support ring. The magnetic counterbalancing means make the motor rotationally operable by counterbalancing thrust hydrodynamic lifting pressure acting on the rotor hub and generated in the single thrust-hydrodynamic pressure bearing when the rotor hub rotates.
Thus a spindle motor, as well as a disk-drive device employing the spindle motor, according to the present invention employs only one thrust bearing, defined between the lower surface of an upper wall section of the rotor hub, and the upper surface of the support cylinder. Therefore, because hydrodynamic thrust-bearing surface area is reduced over that in conventional motors, fluid friction due to the lubricant is reduced, improving the electrical efficiency of the motor. Moreover, since the only one hydrodynamic thrust bearing between the lower surface of the rotor hub and the upper wall portion of the support cylinder is formed without using a thrust plate, the axial height of the motor thus is reduced.
Moreover, in a disk-drive motor rotating on a hydrodynamic radial bearing and a magnetically counterbalanced single hydrodynamic thrust bearing according to the present invention, the underside surface of the rotor hub is employed in lieu of a thrust plate/member in configuring the lone hydrodynamic thrust bearing. Accordingly, in contrast to conventional disk-drive motor configurations, the end surface of the rotor hub itself is employed as a component of the hydrodynamic thrust bearing.
Three parts constitute the hydrodynamic bearings in a motor according to the present invention, namely: the rotor hub, the shaft, and the support cylinder. The hydrodynamic bearings include bearing gaps defined by four surfaces, namely: the lower surface of the rotor hub upper wall portion; the outer circumferential surface of the shaft; and the upper and the inner circumferential surfaces of the support cylinder. Therefore, motor configurations according to the present invention enable motor manufacturing costs to be reduced.
In sum, a disk-drive motor according to the present invention comprises one single hydrodynamic thrust bearing, configured between the underside of the rotor hub and the adjacent end face of the support cylinder. Configuring the motor to have unilaterally a lone thrust-hydrodynamic pressure bearing supporting the shaft necessitates counterbalancing means to achieve the axial balance essential to make the motor rotationally operable. Rather than another thrust-hydrodynamic pressure bearing, the reduced axial-height configuration according to the present invention employs the magnetic counterbalancing means.
Thus, no hydrodynamic thrust plates are employed in a spindle motor/disk-drive device embodied according to the present invention, and the motor/disk-drive device can be made smaller and reduced in axial height. Moreover, since hydrodynamic thrust plates must be manufactured within close tolerances and precise assembly techniques, the spindle motor of the present invention that lacks a hydrodynamic thrust plate entirely is easier to manufacture and assemble, thereby making the manufacturing process less costly. In the motor of the present invention, thrust loads are balanced by the single thrust bearing in combination with magnetic attraction whereby the single thrust bearing supports the rotor hub in a first axial direction and the magnetic attraction supports the rotor hub in a second axial direction.
A hard disk driving device according to the present invention utilizing the motor of the present invention described above, can be made thinner, smaller, less costly to manufacture, and capable of rotating a hard disk with precision.
From the following detailed description in conjunction with the accompanying drawings, the foregoing and other objects, features, aspects and advantages of the present invention will become readily apparent to those skilled in the art.